Epicentres of Blackpool earthquakes (yellow star). The location of the Preese Hall drill site is shown by the blue triangle. The red triangles show in locations of temporary monitoring stations installed by BGS. CLICK TO ENLARGE

In Lancashire, UK, 58 earthquakes were linked to fluid injection
during hydraulic fracturing at the Preese Hall well in 2011 (de Pater
and Baisch, 2011). The largest, on 1 April 2011, had a magnitude of
2.3 and was felt locally. These hydraulic fracture treatments were
carried out during exploration of a shale gas reservoir in the Bowland
basin, Lancashire. A further magnitude 1.5 ML earthquake was felt on
27 May, 2011 and also linked to hydraulic fracture treatments, leading
to the suspension of operations at Preese Hall.

The unusual seismicity led to a number of detailed studies of the relationship
between the earthquakes and hydraulic fracturing operations (for
example, de Pater and Baisch 2011; Eisner et al., 2011). In total, 58
earthquakes were detected in the time period between 31 March and 30
August 2011, nearly all of these either during or within a few hours
of fracturing operations at Preese Hall. De Pater and Baisch (2011)
concluded that the earthquake activity was caused by fluid injection
directly into a nearby fault zone, which reduced the effective normal
stress on the fault and caused it to fail repeatedly in a series of
small earthquakes. A possible causative fault was later identified
following a detailed 3-D seismic reflection study (Clarke et al,
2014).

Correlation between earthquakes and injected volume

The volume of injected fluid (blue line) and earthquakes (red circles,
scaled by magnitude) during hydraulic fracture operations at the
Preese Hall well, Blackpool, between March and June 2011. There are
five distinct treatment stages. Earthquake activity closely correlates
with stages 2 and 4. The largest event with a magnitude of 2.3 ML
occurred shortly after stage 2. CLICK TO ENLARGE.

Earthquake occurence strongly correlates with the injection of fluid
during and immediately after hydraulic fracture stages 2, 4 and 5, in
which the largest amount of fluid was injected. In two of the
hydraulic fracture treatments, stages 2 and 4, the largest earthquakes
occurred approximately ten hours after the start of injection, while
the well was shut-in under high pressure. These events were preceded
by smaller events, which started immediately after injection, the
largest of which was a magnitude 1.4 ML event on 31 March.

No seismicity was observed during stages 1 and 3, and only very weak
seismicity occurred during stage 5. The lack of seismicity in stage 3
can be attributed to the smaller pumped volume and aggressive
flowback. The pumped volume in stage 5 was similar to stages 2 and 4,
but there was also flowback, which could explain the lack of larger
events. The results show that injected volume and flowback timing are
an important controlling factor in the level of seismicity, as
evidenced from the lack of seismicity during and after stage 3,
suggesting that seismicity can be mitigated by modifying job
procedure.

Seismicity began close to the start of injection during stages 2, 4
and 5. This continued throughout injection, and, in the case of stages
2 and 5 for several hours after shut-in. The largest earthquakes
occurred approximately ten hours after shut-in, when earthquakes with
magnitudes of 2.3 ML and 1.5 ML were observed following stages 2 and 5
respectively. Very few earthquakes were detected following these two
events. The lack of seismicity during stage 3 can perhaps
be explained by the reduced injection volume and the use of
flowback. The use of flowback may also explain the relatively weak
seismicity observed during stage 5. The reason for the lack of
seismicity during stage 1 is unclear.

Earthquake Locations

Locations for the Blackpool earthquakes were determined by Eisner et
al. (2011) and Clarke et al (2013) among others. Similarity between
the recorded events suggested that all the events were from the same
location and had the same mechanism. It is clear that the location is
less than 0.5 km from the well head. In addition, the depths of 3.6 km
and 2.9 km, estimated by Eisner et al. (2011) and Clarke et al (2013)
are close to the point of injection (2.3 to 2.7 km)for all 6 stages.

The yellow star shows the epicentre for the Blackpool
earthquakes in April and May, 2011, as determined by Eisner et
al. (2011). The coloured triangles in (a) show permanent monitoring
stations operated by the British Geological Survey at epicentral
distances of 75 to 99 km (red), 100 to 149 km (orange) and greater
than or equal to 150 km (yellow). The red triangles in (b)show
temporary stations deployed after the initial earthquakes on 1 April
2011. The blue triangle shows the location of the Preese Hall well.
CLICK TO ENLARGE.

Is the Earthquake Activity at Preese Hall Unique

Hydraulic fracturing or fracking involves the injection of sand and chemicals at high pressures, causing networks of fractures to open. The sand particles hold the cracks open to allow gas to flow out. CLICK TO ENLARGE

It is relatively well-known that anthropogenic activity can result in
man-made or "induced&quot earthquakes. Although such events are generally
small in comparison to natural earthquakes, they are often perceptible
at the surface and some have been quite large. Underground mining,
deep artificial water reservoirs, oil and gas extraction, geothermal
power generation and waste disposal have all resulted in cases of
induced seismicity. Davies et al (2013) presented a review of
published examples of earthquakes induced by a variety of
activities. There are numerous examples of induced earthquakes in
hydrocarbon fields related to oil and gas production (e.g. SUCKALE, 2010). These are often a response to long-term production, where the
extraction related subsidence is compensated by, for example, normal
faulting on existing faults near or inside the reservoir (Van Eijs et
al., 2006). For example, in 2001 a magnitude 4.1 Mw earthquake
occurred in the Ekofisk field in the central North Sea (Ottemoller et
al., 2005). The earthquake was thought to be related to the injection
of around 1.9x106 m3 of water.

Induced earthquakes with magnitudes as large as 3.5-4.0 ML are well
documented in Enhanced Geothermal Systems (EGS)
(e.g. MAJER ET AL, 2007), in which injected fluids
are heated by circulation through a hot fractured region of
crystalline rocks and then brought back to the surface for power
generation. A series of magnitude 3+ earthquakes induced during an EGS
project in Basel, Switzerland resulted in the suspension of the
project, which was ultimately abandoned almost 3 years later following
further study and risk evaluation after these seismic events
(Giardini, 2009).

Earthquakes can also be induced by the injection of brines from oil
and gas production into wells that are drilled to dispose of large
volumes of waste water over many years (e.g. Frohlich et al,
2011). Induced seismicity caused by long term disposal of large
volumes of waste fluid in deep boreholes is suspected to be partially
responsible for a significant increase in seismicity rate that has
been observed in Eastern North America (Ellsworth, 2013). In addition,
several of the largest earthquakes in the U.S. midcontinent in 2011
and 2012 may have been triggered by nearby disposal wells
(e.g. Horton, 2012; Kim, 2013), suggesting that wastewater disposal by
injection to deep wells poses a significant seismic risk. The largest
of these was a magnitude 5.7 event in central Oklahoma that destroyed
14 homes and injured two people (Kerenan et al, 2013).

The process of hydraulic fracturing in order to increase the
permeability of reservoir formations and stimulate the recovery of
hydrocarbons is generally accompanied by microseismicity, commonly
defined as earthquakes with magnitudes of less than 2.0 and too small
to be felt. The mechanisms for this are generally well
understood.

Firstly, the injection of fluids under high pressure
generates new cracks and fractures in a previously intact rock
mass. As these grow and spread they are accompanied by brittle failure
of the rock and corresponding microseismic events. These are sometimes
referred to as "fracked&quot events. The size of these
"fracked&quot events is constrained by the energy of the
injection process.

Secondly, both presence of high pressure fluid and
the stress perturbation caused by the fluid can change the effective
stress on pre-existing faults, causing them to fail. These events are
sometimes referred to as "triggered&quot events. Since small
stress perturbations can cause relatively large earthquakes the size
of these events depends largely on the amount of stored up elastic
strain energy already in the rocks.